2Department of Plant Protection, Faculty of Agriculture, University of Tabriz, Tabriz, Iran.

Abstract

Colorado potato beetle, Leptinotarsa decemlineata is one of the major insect pests of potato. Toxicity of spinosad, as a bio-rational insecticide, was investigated against various developmental stages of this pest. Bioassays were conducted by using the eggs, neonates, first, second, third and fourth instar larvae and adults. The potato leaves were impregnated with different concentrations of spinosad and applied for the adults and different larvae bioassays. The eggs were tested through dipping its masses into the insecticide solutions. LC50 values of neonates, first, second, third and fourth instar larvae and unsexed adults after 24 hours were 2.06, 3.19, 4.75, 6.46, 20.24 and 11.97 ppm (of commercial formulation), respectively. Results show that spinosad did not possess any ovicidal effects and the fourth instar larvae and neonates were the most tolerant and the most susceptible stages, respectively. Susceptibility of the neonates (up to 24 hrs after hatching) was significantly higher than that of first instar larvae (24-48 hrs after hatching). Developmental stages of Colorado potato beetle responded differentially to this insecticide. Since the control of L. decemlineata mostly relies on early season measures against the most susceptible stage(s), by considering no ovicidal effect, our results propose a limited interval, for avoiding the highly tolerant larvae.